Abstracts for Michael A. Lieberman

The EECS Research Summary for 2003

Characterization of Instabilities in Inductively Coupled Plasmas

We study plasma instabilities in inductive discharges using SF6 and Ar/SF6 gases. Instabilities occur in transition from capacitive to inductive mode of the discharge [1].
The capacitive coupling plays a crucial role in the instability process. A variable electrostatic (Faraday) shield has been used to control the capacitive coupling from the excitation coil to the plasma. An increase of the shielded area reduces the capacitive coupling and leads to the reduction of stable capacitive and unstable regions of the discharge. The plasma instability disappears when the shielded area exceeds 65 percent of the total area of the coil. A global model of instability gives a slightly higher value of 85 percent for instability suppression with the same discharge conditions (Ar/SF6 1:1, 5 mTorr).
The variations of plasma parameters have been explored for Ar/SF6 gas pressures between 5 and 40 mTorr. Space and time variations of plasma density have been observed at higher pressures during the instability. The instability decays toward the center of the chamber more rapidly as the gas pressure increases.
Weaker plasma instabilities have been also observed in pure inductive discharges for an Ar/SF6 gas pressure above 20 mTorr. This weaker instability, not related to the E-H transition, is probably an attachment instability. Measurements of the chemical composition of the plasma have allowed improvements in the instability model.

Modeling Dual Frequency Capacitive Discharges*

As packing density increases in the fabrication of semiconductors, the fine control of dry etching is strongly required for transferring the fine patterns from weak and thin photo resist. The dry etching is well known to be controlled by ion energy, ion flux, radical flux, and radical species. These control parameters are strongly related to the electron temperature, electron density, and sheath potential of the bulk plasma. In our research, independent control of electron temperature, electron density, and sheath potential of the bulk plasma will be demonstrated through simulations of a circuit model and a global model for the CW and pulse modulated dual frequency capacitively coupled plasma (CCP). This research is focused on the roles of frequency, electrode gap, shape of pulse, chemistry, etc.

Global Modeling of Time Modulated Electronegative Plasmas

In this research, we are developing a model to investigate the properties of electronegative plasma driven by time modulated power. We have started with a study on the O2 plasma to develop a general model for electronegative discharges. The calculations are done utilizing a global model of a cylindrical plasma discharge [1,2]. This model assumes uniform spatial distribution of plasma parameters over the volume of bulk plasma, with the plasma density in the bulk dropping sharply to edge values at the walls. The global model consists of particle balance and power balance equations. In the particle balance equations, the reaction rates of O2+, O- ions, and electrons have been included for simplicity. The O+ ions and metastable O atoms will also be included later. We are developing a simulation code that can be used in the simulation with variable power modulation periods, duty ratios, and modulation pulse shapes.

As a preliminary examination of the simulation codes, calculations for a sinusoidal modulation of power with a duty ratio of 50% have been performed. The pulse period was varied from 1 microsecond to 1 millisecond. In the shorter modulation period, we observed that the electron temperature falls rapidly during the off cycle whereas the plasma density does not. And the time-average plasma density and the time-average electron temperature showed dependency on the modulation period.

We are working on the optimization of the simulation codes to include more reaction chemistry and are planning to do more simulation with various duty ratios or modulation shapes. Later we will apply the results from this research to the modeling of dual frequency discharge.

Figure 1: Plasma densities averaged over one period of the modulated power with various modulation periods.
The dotted line represents plasma density of the CW driven plasma.
Figure 2: Electron temperatures averaged over one period of the modulated power with various modulation periods.
The dotted line represents electron temperature of the CW driven plasma.

Dual frequency discharges give independent control of ion bombarding energy and plasma density. A high frequency RF source, between 20-80 MHz, controls the plasma density, while a low frequency RF source, between 1-20 MHz, sets the sheat bias and ultimately the ion impact energy. Particle-in-cell (PIC) simulations will be used to characterize the performance of such devices, including kinetic and nonlinear effects. Collisions with background gas are modeled with a Monte Carlo collision (MCC) model, including electron-neutral scattering, excitiation, ionization, and ion-neutral scattering and charge exchange.

One dimensional cylindrical simulations run in a matter of hours, and allow asymmetric electrodes. Preliminary PIC-MCC results compare favorably with global models. Two dimensional simulations are planned to correctly model the grounded third electrode (wall) of realistic ion etch reactors.